1. Field of the Invention
The invention relates to methods of polymerizing rosin.
2. Brief Description of the Prior Art
The term "polymerized rosin" refers to the resinous mixture obtained when rosin is treated under various conditions with strong inorganic acids or acidic clays. The mixture comprises non-reactable rosin acids such as dehydroabietic acid, rosin acids containing some unsaturation which did not react and a number of different types of polymerized rosin acids including dimerized rosin acids. The mixture also contains a minor amount of esters, rosin anhydrides and non-saponifiable substances. Polymerized rosin may be refined, usually by distillation, to remove some portion of the monomeric rosin acids and other substances to produce a mixture containing a higher concentration of polymerized rosin acid. This refined mixture always has a higher softening point than unrefined polymerized rosin and it is referred to as "dimerized rosin" in many textbooks.
There are many references in the chemical literature which describe the utility of polymerized rosin. The "Encyclopedia of Chemical Technology", 2nd edition, Vol. 17 p.497-498, the "Encyclopedia of Polymer Science and Technology", Vol. 12, p. 139-161 and the U.S. patents of Rummelsburg (U.S. Pat. Nos. 2,136,525 and 2,108,928) describe the numerous and varied applications for polymerized rosin. It is pertinent to note that all of the applications for this valuable substance may be divided into two categories; those in which it is reacted chemically such as in alkyds, polyesters or resinates and those in which it is used by means of physical admixture such as in a solvent born mixture with elastomers to prepare pressure sensitive adhesives, in hot melt adhesives where it is also used in combination with high polymers as a molten blend and in solvent born mixtures with high molecular weight coating polymers such as nitrocellulose. Dimerized rosin may be regarded as a high quality polymerized rosin which will usually exhibit improved properties advantageous in almost all of the chemical and physical blend uses. An improved physical property common to all polymerized rosin (which is usually a major reason for its utility in physical blends) is the higher degree of resistance of polymerized rosin to crystallize from solvent solution or from the molten state. In their paper, "The Relationship of Dimer Content of Rosin Determined by Gas Chromatography to Crystallization Time", Leonard et al, (Journal of The American Chemical Society, Vol. 42, p. 111 to 113, 1965) observed that this property can be treated quantitatively, and that in general, the higher the dimer rosin content, the greater the resistance of the polymerized rosin to crystallization. This was confirmed by Sinclair and Schuller et al, reporting in "Influence of Reaction Conditions on the Dimerization of Abietic Acid and Rosin", Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970. Their data provides an excellent reference for the analysis of polymerized rosin and a criteria for assessing the efficiency of any polymerization process. That criteria is simply the content of dimer rosin developed in a single treatment. However, dimer content alone does not entirely account for the degree of crystallization resistance seen in a given mixture of polymerized rosin.
Tall oil rosin is one of the most available of all rosin types but it also has the greatest tendency to crystallize as shown by Leonard et al. Supra. Any process which is used to polymerize rosin could theoretically be used as a partial or full treatment to decrease the tendency of tall oil rosin to crystallize and thus give it greater utility. Unfortunately, none of the processes known until now are very convenient. Much prior art exists that bears upon the conversion of rosin, including tall oil rosin, to a more polymeric substance. The patents of Rummelsburg mentioned above in addition to his more recent U.S. Pat. Nos. 2,307,641 and 2,328,681 are examples, also the patent of Hampton U.S. Pat. No. 2,515,218. Also representative of the prior art are U.S. Pat. Nos. 2,251,806; 2,532,120; and 4,105,462.
In spite of the many methods which have been described for polymerizing rosin, there are at least two disadvantages which usually attend all processes for polymerizing rosin. First, the nature of the acid catalyst is a disadvantage, ie; its acidity makes it difficult to handle. All of the catalysts known to be operable within a reasonable process time to obtain acceptable yields of dimer suffer from some combinations of the following disadvantages. They are corrosive; they evolve corrosive or poisonous fumes; their salts are toxic; they need be used at high levels; they leave residuals (often chemically bonded contaminants) in the final product; they require processing of rosin at relatively low temperatures; and they require processing of rosin in the presence of large amounts of inert solvent. In regard to the latter two points, rosin when used in production quantities is usually stored in the molten condition at temperatures around 150.degree. C. To dissolve it in solvents for processing at 30.degree. to 60.degree. C. requires considerable cooling (a time consuming step). The rosin may crystallize in the process and all the solvent must be stripped out later (which results in low kettle yields). The second major disadvantage of prior art processes, particularly those which use sulfuric acid, is the separation of an acidic sludge or emulsion during work-up. Tall oil rosin polymerization is especially prone to develop a large acid sludge layer which can be very troublesome. The problem is mentioned specifically by Sinclair et al in U.S. Pat. No. 3,663,545.
the present invention is an improvement over the prior art in that it permits one to attain the highest content of polymerized rosin possible in a fast, single step using a convenient manufacturing sequence. The product, having a high dimer acid content, resists crystallization for moderate periods of time without a need for further refining to remove monomer acids and other materials which may promote crystallization.
The product polymerized rosin also has a color which is generally no darker than I (Rosin Cube) and has a softening point equal to or greater than the starting rosin material.
A substantial advantage of the method of the invention resides in the class of acid catalyst used to promote the polymerization. This class of acids avoid practically all of the disadvantages of other previously known strong acid catalysts. They are homogeneous catalysts which dimerize even tall oil rosin without the formation of acid sludge. They operate at higher temperatures (in the range of 60.degree. to 180.degree. C. but preferably in the range of 90.degree. C. to 150.degree. C.), they can be used at levels as low as 0.01% of acid based on rosin (a level of 0.3% to 1.0% is preferred) but even a level of 3.0% or more can be used in a fast, low temperature process. It is surprising that water does not deactivate these catalysts. This is advantageous and it is a part of the invention that they may be supplied to the reaction as a solution of 50% or more in water. This is fortunate because these catalysts are much more convenient to synthesize in water solution than they are in the anhydrous state. These and other advantages will be described more fully hereinafter.